I turned off the terrain rendering and that made a big difference, so that confirms much of the rendering cost is associated with terrain. Interestingly, I've tried smaller heightmaps, but I feel like there wasn't much difference.

The water isn't mobile friendly, and turning that completely off helped too. But obviously, the terrain and water are important for the game, so I can't just remove them.

I think I've decreased the quality settings as much as possible. It looks like crap compared to the PC version. But it's at least playable now.